While I had the car down I was muttering around in the engine bay replacing vacuum lines and i stumbled across something that appears to have been modified from stock. I have a mysterious vacuum part on the passenger side just in front of the firewall. It kind of looks to me like the line that went to it maybe now goes into that T above and behind it instead? Both of the larger lines on the T headed for the intake manifold/ plenum that would be on the car if it would have been there instead of laying on the floor of the garage.

I obviously have lived without it because it's been like that for the last 3 years at least but i wouldn't mind knowing what I'm missing out on


Thanks for any info.

 

Just the warmest and coldest possible air in the a/c system.

Thats the heater control valve and the a/c system shuts off hot water inside the core when the a/c is used. It opens the flow again when ehat is needed. This gives the most efficient cabin air.

 

You were all over that one I must try harder to challenge you in the future, thanks. Next question is why would someone bypass it? If it's sticking in or out it would mess with the heater or air conditioning? I'll have to look around-it doesn't appear to be an expensive part and maybe can be fixed in some manner.

I've been thinking about trying to get the air to work right and this looks like a step in the right direction.

 

well....it may leak when its thrown over to the other position. When the core is full of hot water there can be some mixing in this small evaporator core and you loose some of the cooling effect of the a/c....how much, I cannot say for certain. Heat is easier since the a/c is easier to warm than it is to cool hot air.
The a/c works by removing hot air from the cabin, so bringing hot air inside the evap box when you;re trying to remove it, does not make much sense. thats why they added this gadget to try to keep the hot water out when the a/c is running. If its like mine, the valve blocks flow to the heater core and sends the water thru a bypass tube/union to the return side of the other heater hose so the water still circulates everywhere except the heater. As the valve opens it allows water to go thru the heater core providing heat for the cabin as needed. Its more expensive than it looks...You can easily make a set of hoses that go to the core without all the broken plumbing but you then have hot water in the core all the time. Or you can just make a loop and bypass the heater core completely.
IF the a/c vent doors seal tight and the mix doors are tight, it might not make any difference. But I suspect that it does to some extent. These vent door seals are foam or felt and gthey dry out with age and the doors do not seal as well...so some hot air gets mixed with your cold air then sent to the vents.

I'm running a bypassed set up now because my heater control valve leaked and the replacement was not available at the time. It probably does reduce the cooling efficiency...I just can;t say how much. When its as miserably hot as this past summer was/has been, every little bit helps.

 

It may be unhooked due to the fact that it may leak when engaged. Mine leaks when I put the heater controls on "hot". There are some threads on how to rebuild it and use a modded one out of a S10 or something. Someone now reproduces it for a good chunk of change. Hook it up and see if it leaks.

 

Lee beat me dammit.

 

Thanks guys-maybe lee just pounds on the keys a little faster.

 

Here is some info I condensed from this Forum a while back. The early year control valve rebuilds are around $130 and they are doing what is detailed below. You can even use the valve without trying to change out the internals but it will take a little extra rubber hose as the C4 one has longer metal tubing.

Heater Control Valve Rebuild Information

The heater control valve can be rebuilt using the internal parts from a more readily available replacement valve. GM used this valve on several models in the same period. The following are two examples.
1982-3 Malibu and a 1982-84 Full Size. Here is a quote from Corvette Forum from someone who had done the rebuild
Original heater control valve part #’s:
1985, 1986, 1987 Corvette heater control valve, hot water valve.
AC Delco - 15-5389
GM – 10120965

Replacement Chevy Valve Part #’s:
This valve fits several mid 80’s GM cars. I bought one from Advance Auto and it matched up perfectly. If you get one of the other brands, just make sure the tab cutouts line up to your old valve before you tear it apart. For a parts lookup you can use:
1982-1983 Chevrolet Malibu V6-229 3.8Lw/Gauges (4 Port valve)
1982-1984 Chevy Full Sized – V8 305 w/Gauges (4 port valve)

ReadyAir – 5935 (Autozone brand)
Four Seasons - 74800
Factory Air – 74800 (Advance Auto brand)
AC Delco - 15-5302

Heater Hose Sizes:
Heater core inlet: 5/8” (upper tube)
Heater core outlet: 3/4" (bottom tube)

 

Good info there

 

I eliminated the valve due to leaking and I can't tell any difference when running AC.
Later years, GM removed the valve.

 

I did the same after one replacement that only lasted days...few weeks at most.
I cannot say for sure that it makes a difference. Perhaps next yr when summer arrives, I'll just bypass the heater core and see....Then again, I've been told that letting the heater core dry out is the fastest way to make it rot out and need replacement...thats a scarey thought.

 

Correct me if I am wrong guys....but doesn't this valve control flow into the heater core? Won't hot air com out of my vents in the summer time if I perform a bypass? Edit: My a/c is non functional.

 

If this is so and it's a big deal maybe remove the thing all together then put a little hand valve in there. If it's cool out then pop the hood and open the valve.

 

Water has to have a way to circulate back to the pump. If the heater core is bypassed the heater hoses must be connected to each other so water flows back. Otherwise water is trapped in the intake and cannot move causing heat problems with the eng temp.The only other way for water to flow is thru the T-stat but that would provide unequal temp control for aluminum parts. Even when bypassing its best to use some type of orifice inline to slow the flow so that it ALL does not flow thru the heater lines. A little back pressure prevents hot/dry spots in the intake and heads and keeps the lines full.

 

Wisdom-maybe I'll just leave mine alone for now and worry about it when i get it out next year.